1. Field of the Invention
This invention relates to a method of determining the downhole flow regime in a horizontal well borehole. In particular, it relates to a method of determining the downhole flow regime in a horizontal wellbore by use of center sample and fullbore gas-liquid holdup measurements.
2. Description of the Related Art
Production logging (PL) data are used to determine the rate and type of fluid entering or leaving the completion string. The PL data can be classified according to the type of measurement used to obtain the data: fluid velocity, holdup, pressure, temperature, and auxiliary measurements, such as noise. The velocity of the composite downhole flowstream is determined from data obtained from multiple passes of an in-line or fullbore spinner or from data obtained from station measurements made with a diverter/basket flowmeter. The downhole flow rates for each phase are determined from the fluid velocity, the holdup of each phase, and the interphase velocity relationship, which in vertical wells is referred to as the slip velocity. The limitations of fluid-velocity measurements in multiphase flow (e.g., gas, oil, and water) in horizontal wells and in unstabilized flow regimes in vertical wells are well known.
Oxygen-activation measurements from multiple-detector pulsed neutron tools are routinely used for water-velocity determination. Oxygen-activation combined with conventional PL data can be used to determine whether the water movement is inside or outside the production tubulars.
At a particular depth, the holdup of a specified phase (gas, oil, or water) is defined as the fraction of the cross sectional area of the casing or tubing that is occupied by that phase. The traditional holdup logging devices are the radioactive fluid-density (gamma-gamma attenuation), the gradiomanometer (differential pressure), and the water-holdup (capacitance, or dielectric) tools. Differential-pressure holdup measurements must be corrected for well deviation and are unusable in horizontal wells. The radioactive fluid-density and capacitance water-holdup tools are, by design, center-sampling tools and have radial depth of investigation approximately equal to the radius of the logging tool. A center-sampling tool thus measures fluid properties along or near the axis of the tool. An example of a typical center-sampling radioactive fluid-density tool used for gas holdup measurements is a fluid density logging tool, model no. FDT-EC, available from Halliburton Energy Services of Houston, Tex.
The identification of gas entry points in high-angle and horizontal wells is often difficult when high liquid holdups are present. In such situations, the liquid phase occupies a large cross-sectional area of the casing, and the gas, because of its lower density occurs in the upper portion of the casing. At low gas flow rates, the gas phase may occur as individual gas bubbles. As the gas flow rate increases, stratified, or layered, flow can be present. Conventional center-sampling holdup tools cannot identify the gas holdup until the gas occupies an area from the upper surface of the casing downward to near the axis of the centralized logging tool where it is within the radial depth of investigation of the center sample measurement.
In horizontal wells, the type of flow regime may be determined by crossplotting superficial gas velocity against superficial liquid velocity on a flow-pattern map as illustrated in FIG. 1. The various flow regimes in the horizontal well are further illustrated in FIG. 2. The reported surface production rates, converted to downhole volumes and corresponding velocities for the gas and liquid phases, can be entered into the map to obtain the flow regime for 100 percent flow conditions. As discussed earlier, center-sampling holdup tools will give incorrect holdup values in horizontal wells in all multiphase flow regimes except dispersed bubble flow (where the gas phase is uniformly distributed within the liquid phase). Superficial velocity is a function of (1) the total fluid velocity, (2) the slip velocity between the phases, and (3) the phase holdup. Therefore, if incorrect holdup values are utilized, it directly follows that the superficial velocity will also be in error and will thus predict an incorrect flow regime.
Fullbore gas holdup tools are now available which overcome the limitations of the traditional center-sampling tools by providing gas holdup values for the entire wellbore rather than simply the central portion of the wellbore. A gas holdup tool for use in cased well boreholes is disclosed in U.S. Pat. No. 5,359,195, the disclosure of which is incorporated herein by reference. The unique design of the fullbore gas holdup tool disclosed in U.S. Pat. No. 5,359,195 permits the measurement of gas holdup values that represent the entire cross sectional area of the wellbore in which the tool is positioned. The full capability of the fullbore gas holdup tool measurements to date has not been fully developed. In particular, fullbore gas holdup tool measurements have not been used in conjunction with more traditional measurements such as those obtained by the older generation of center sampling gas holdup tools to derive downhole flow regimes.
The present invention is directed to a method for determining the downhole flow-regime that overcomes the limitations of prior methods that relied solely upon center-sampling gas-liquid holdup measurements. In particular, the present invention is directed to a method for determining the downhole flow-regime by utilizing a combination of center sample and fullbore gas holdup measurements.